System and method for utility and wind turbine control

ABSTRACT

An exemplary embodiment includes a wind turbine system. The wind turbine system includes a wind turbine generator operable to supply wind turbine power to a utility system. A converter is coupled to the wind turbine generator and the utility system. The wind turbine system also includes a controller comprising an internal reference frame of the wind turbine generator, coupled to the converter, and configured for modulating flow of power through the converter in response to frequency disturbances or power swings of the utility system relative to the internal reference frame.

BACKGROUND

The invention relates generally to the field of wind turbine generatorsused for power generation for utilities, and more particularly totechniques for stabilizing power during transient conditions.

Wind turbine generators are regarded as environmentally friendly andrelatively inexpensive alternative sources of energy that utilize windenergy to produce electrical power. A wind turbine generator generallyincludes a wind rotor having turbine blades that transform wind energyinto rotational motion of a drive shaft, which in turn is utilized todrive a rotor of an electrical generator to produce electrical power.Modern wind power generation systems typically take the form of awind-farm having multiple such wind turbine generators that are operableto supply power to a transmission system providing power to a utilitysystem.

These wind turbine generators and wind-farms are typically designed todeliver power to the utility system with the power being independent ofsystem frequency. Some wind turbine generators have a variable frequencyoperation and require a variable frequency power electronic inverter tointerface the wind turbine generator output with the utility grid. Inone common approach the wind turbine generator output is directly fed toa power electronic converter, where the turbine frequency is rectifiedand inverted into a fixed frequency as needed by the utility system. Analternative approach uses a doubly fed asynchronous generator (DFAG)with a variable frequency power electronic inverter exciting the DFAGrotor and stator windings being coupled directly to the utility system.

In traditional power systems, the frequency of the synchronousgenerators of the power system match the utility system, and the dynamicresponse of the frequency of the utility system is dependent upon theinertia of the synchronous generators and loads. Synchronous generatorsused in a traditional power system are able to contribute in frequencyand voltage control of the power system during transient conditions,that is, sudden failure of generation, line fault or connection of alarge load. During of transient conditions, the system frequency startsto change at a rate mainly determined by the total angular momentum ofthe system. The total angular momentum is a sum of the angular moment ofall the generators and rotating loads connected to the power system. Insuch transient conditions, the synchronous generators may also provideadditional control services that modulate active power to stabilize thepower system and restore frequency to its nominal value.

Wind turbines, when used for generating power in a power system,however, do not contribute to the frequency stabilization of the utilitysystem. As more power generated by wind turbines is interfaced throughthe utility system, it would be desirable for wind turbines to alsocontribute to the voltage and frequency control of the power system intransient conditions in order to stabilize the power system.

Gonzalo Costales Ortiz et al., WIPO Application No 03023224, describes asystem for using turbine mechanical inertia for dynamic stability andfrequency control. The system uses a fixed frequency reference and thederivative of frequency to calculate the supplemental torque and poweroutput to the system. Derivative terms in control systems are subject tonoise that may affect performance. A fixed reference is a difficulty inembodiments wherein the turbine control is desired to track the normalfluctuations in utility frequency without undue supplemental torque orpower interactions.

Therefore there is a growing need to overcome the above mentionedlimitations for wind turbine systems and to provide control techniquesso that the wind turbines can participate in frequency regulation andpower-swing stabilization for the utility system.

BRIEF DESCRIPTION

An exemplary embodiment includes a wind turbine system. The wind turbinesystem includes a wind turbine generator operable to supply wind turbinepower to a utility system. A converter is coupled to the wind turbinegenerator and the utility system. The wind turbine system also includesa controller comprising an internal reference frame of the wind turbinegenerator, coupled to the converter, and configured for modulating flowof power through the converter in response to frequency disturbances orpower swings of the utility system relative to the internal referenceframe.

Another aspect of the invention includes a method for stabilizingfrequency and power swings of a utility system. The method includessupplying power from a wind turbine generator to the utility system, andusing an internal reference frame of the wind turbine generator formodulating flow of power from the wind turbine generator in response tofrequency disturbances or power swings of the utility system relative tothe internal reference frame.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a diagrammatical representation of a wind turbine system forstabilizing power and frequency during transient conditions for autility system in accordance with one embodiment;

FIG. 2 is a diagrammatical representation of a control loop used by thecontroller of FIG. 1 to provide a supplemental input signal in order tostabilize power and frequency during transient conditions in accordancewith one embodiment;

FIG. 3 is a graphical representation of exemplary power limits enforcedwhile generating the supplemental input signal in the control loop ofFIG. 2;

FIG. 4 is a simulated graphical representation of frequency deviation ordisturbance response for an oscillatory system disturbance with andwithout the control loop as shown in FIG. 2;

FIG. 5 is a simulated graphical representation of the supplemental powerdisturbance response for an oscillatory system disturbance with andwithout the control loop as shown in FIG. 2;

FIG. 6 is a simulated graphical representation of frequency deviation ordisturbance response for a monotonic oscillatory system disturbance withand without the control loop as shown in FIG. 2;

FIG. 7 is a simulated graphical representation of the supplemental powerdisturbance response for a monotonic system disturbance with and withoutthe control loop as shown in FIG. 2; and

FIG. 8 is a diagrammatical representation of a wind farm managementsystem for stabilizing power and frequency during transient conditionsin accordance with one embodiment.

DETAILED DESCRIPTION

Referring generally to FIG. 1, a wind turbine system 10 operable togenerate electric power is provided. The wind turbine system 10comprises a hub 12 having multiple blades 14. The blades 14 convert themechanical energy of the wind into a rotational torque, which is furtherconverted into electrical energy by the wind turbine system 10. The windturbine system 10 further includes a turbine portion 16 that is operableto convert the mechanical energy of the wind into a rotational torqueand a generator 18 that is operable to convert the rotational torqueproduced by the turbine portion 16 into electrical power. A drive train20 is provided to couple the turbine portion 16 to the generator 18. Thewind turbine generator 18 typically comprises a doubly fed asynchronousgenerator or a full conversion synchronous generator or a generator foruse with a full converter. In a full conversion embodiment, the windturbine generator stator windings (not shown) are directly fed to theconverter. In a doubly fed embodiment, the generator rotor windings (notshown) are coupled to the converter and the generator stator windings(not shown) are coupled directly to the utility system.

The turbine portion 16 includes a turbine rotor low-speed shaft 22 thatis coupled to the hub 12. Rotational torque is transmitted from therotor low-speed shaft 22 to a generator shaft 24 via the drive train 20.In certain embodiments, such as the embodiment illustrated in FIG. 1,the drive train 20 includes a gear box 26 transmitting torque from alow-speed shaft 22 to a high speed shaft 30. The high speed shaft 30 iscoupled to the generator shaft 24 with a coupling element 28. As thespeed of the turbine rotor low-speed shaft 22 fluctuates, the frequencyof the output of the generator 18 also varies. In one implementation ofthe above embodiment, the transient overload capability of the windturbine electrical and mechanical systems at full load is utilized bydecreasing blade pitch and/or turbine speed to transiently increasepower. The degree and duration of this overload are managed such thatundue stress on the mechanical and electrical system components isavoided.

In one exemplary embodiment, the generator 18 is coupled to wind turbinecontrols 44. The wind turbine controls 44 receive signals 46 from thegenerator that are representative of the operating parameters of thegenerator. The wind turbine controls 44 in response may generate controlsignals, for example a pitch signal 56 to change the pitch of the blades14. The wind turbine controls 44 are also coupled to a controller 32having an internal reference frame, described in more detail inreference to FIG. 2. The controller 32 is coupled to a converter 34. Theinput 48 from the wind turbine controls 44 and the input 50 from thecontroller 32 are summed in a summation element 52 and is supplied asinput 54 to the converter 34. The converter 34 typically includes powerelectronics components to convert the variable frequency output 36 ofthe generator 18 into a fixed frequency output 38 for supply to autility system or a power grid 40. The wind turbine controls 44,controller 32 and converter 34 are described in more detail withreference to FIG. 2.

The controller 32 is configured for modulating flow of power through theconverter 34 in response to utility system frequency disturbances orpower swings relative to the internal reference frame. The controller 32is also coupled to the utility system 40 and receives input signals 42from the utility system 40. The signals 42 may be representative of theutility system parameters, for example frequency or power.

Also shown in FIG. 1 is block 60 representative of optional energystorage elements, optional energy consumer elements or combinationsthereof. Energy storage elements may comprise elements such asbatteries, capacitors, and flywheels, for example. Energy consumerelements may comprise loads or dissipative resistors, for example. Suchelements may optionally be controlled by a local converter controller(within converter 34) or by another controller if desired. For examplethe flow of power in at least one energy storage element or an energyconsumer element may be modulated in response to frequency disturbancesor power swings of the utility system relative to the internal referenceframe

FIG. 2 is a diagrammatic illustration of an exemplary control loopemployed in the controller 32. The controller 32 provides a supplementalinput signal to the converter 34, which may comprise a power or torquesignal and is denoted generally by reference numeral 50 and symbol ΔP.It may be noted that power and torque are used interchangeably in thedescription herein. As discussed in more detail below, the supplementalinput signal ΔP is typically a function of at least one of relativeangle, relative frequency, or time with respect to the utility systemand the internal reference frame. The supplemental input signal ΔP isexpected to lead to an increase or decrease in power output of the windturbine system to stabilize the overall utility system. ΔP is zero whenthe internal reference frame matches the utility system frequency andangle during steady-state conditions. Under transient conditions, if thesystem frequency or angle relative to the internal reference frame isdecreasing then ΔP needs to be increased to enhance stable operation.Similarly, if the system frequency or angle relative to the internalreference frame is increasing then ΔP is decreased to enhance stableoperation of the utility system. Further, the supplemental input signalΔP may be continuous or discrete and may be implemented as a closed oropen loop function, subject to certain system limits as discussed below.

Referring back to FIG. 2, a torque or power command signal 48 from windturbine controls 44 may also be provided as an input to the converter34. The supplemental input signal ΔP and the command signal 48 may besummed in the summation element 52. Converter 34 typically includes alocal converter controller (not shown) for converting the inputs intoconverter switching signal commands. In another embodiment, thesupplemental input signal 50 is fed into wind turbine controls 44, andsummation element 52 is included within wind turbine controls 44. Suchembodiments provide additional flexibility. For example, additionallimit functions may be inserted between summation element 52 andconverter 34. In still another embodiment, summation element 52 isoutside of wind turbine controls 44, and the supplemental input signal50 is fed into the wind turbine controls 44 in addition to being fed tothe summation element 52. When the supplemental input signal 50 isprovided to wind turbine controls 44, there is an option to use thesupplemental input signal 50 for feed forward control of features suchas blade pitch or turbine speed control, for example. It may be noted,that the wind blade pitch control signal or the turbine speed controlsignal may be provided in response to the frequency disturbances or thepower swings of the utility system relative to the internal referenceframe. In still another embodiment, the supplemental input signal 50 canbe used to modulate optional energy storage elements, optional energyconsumer elements or combinations thereof (shown as element 60 in FIG.1).

The controller 32, as described above, uses a control technique thattransiently increases power output as a function of relative angle, orrelative frequency between the utility system 40 (shown in FIG. 1) andthe internal reference frame 62 of the wind turbine generator. Thecontroller 32 is thus configured for modulating flow of power throughthe converter 34 in response to frequency disturbances or power swingsof the utility system relative to the internal reference frame 62. Theinternal reference frame 62 is implemented as an integrator in anexemplary embodiment that emulates a “virtual inertia” with a magnitudedefined by the constant “M”. The internal reference frame 62 has anoutput 64 that is variable and is the frequency of the internalreference frame ω_(i). During steady-state conditions the frequencyoutput 64 of the internal reference frame 62 will equal the frequency ofthe utility system. The frequency output 64 may differ from the utilitysystem during frequency disturbances.

The relative frequency {circumflex over (ω)}, donated by referencenumeral 66 is obtained from difference element 68 as a difference ofmeasured frequency ω_(m) (measured utility system frequency) depictedgenerally by reference numeral 70 and the frequency of rotation of theinternal reference frame ω_(i). The relative frequency {circumflex over(ω)} is delivered to block 72, where it is adjusted by the basefrequency ω_(b) and integrated to generate an angle {circumflex over(δ)}, depicted generally by reference numeral 74 that is a relativeangle with respect to the internal reference frame 62. The relativeangle {circumflex over (δ)} is thus calculated from the integral of therelative frequency {circumflex over (ω)} multiplied by a constant basefrequency ω_(b) to convert per unit frequency to radians.

A supplemental control input 76 may be used as an optional input to theinternal reference frame 62 in one example to add the control featuresof power droop with frequency. Feedback loop 78 is provided to adjustthe internal reference frame output as a function of the change inturbine power or torque, which may be combined at summation element 80with supplemental control input 76. For the exemplary embodiment, thisfeedback loop will emulate the inertial effect on internal referenceframe frequency due to changes in power output. In an optional closedloop embodiment, the feedback loop 78 may be derived from the differencebetween turbine control torque or power command 48 and measured turbinepower, which may further optionally be combined at summation element 80with supplemental control input 76.

The controller 32 is further configured to employ a torque or powertransfer function 82, in one example to generate the supplemental inputsignal ΔP. In a more specific embodiment, wherein transfer function 82is a function of both relative angle and relative frequency, therelative frequency is modified by damping element D, shown generally byreference numeral 84, the relative angle is modified by a torque orpower constant, the Kd element, shown generally by reference numeral 86,and the sum is provided at summation element 88 to obtain supplementalinput signal ΔP. As discussed above, an exemplary implementation alsoaccommodates additional energy storage and energy dissipative elements.

A limit function 90 is additionally employed in an exemplary embodimentfor limiting the relative angle 74, an internal reference framefrequency 64, a power or torque signal 50, or any combinations thereof.Although a single block 90 is illustrated for purposes of example, oneor more functions or controllers may be used to implement limit function90 if desired. Limits are useful because, when the wind turbinegenerator is operating at or near rated power output, then an increasein power will tend to overload the generator and converter. The limits92, 94 or 96, used by the limit function 90 may be absolute limits,time-dependent limits, or combinations thereof. Some non-limitingexamples of the limits used by the limit function 90 include physicallimitations on the wind turbine system, power limits, torque limits,ramp rate limits, energy limits, and rotor speed limits of the windturbine generator. Examples of physical limits include thermalcapability of the power conversion equipment, converter current limitsand drive shaft mechanical stress. Examples of energy limits includeenergy storage and dissipative energy limits.

Further there may be specific upper limits and lower limits for systemstability. An upper limit used by the limit function 90 is typically afunction of one or more of the following: converter thermal conditions,loading history, time and even ambient temperature. The lower limit willtend to be symmetric compared to the upper limit, although it is notrequired to be so. Further the limit function can be a limit on theoutput of a control block, or a limit or deadband on the input to acontrol block. The deadband limit is type of limit where over some bandaround zero there is no action and beyond a threshold an action isrequired to accommodate the limit. Some exemplary limits employed by thecontroller 32 are described in more detail below and with reference toFIG. 3.

As a specific example, since the total energy balance on the windturbine dictates the drive-train speed, the energy balance may be usedto determine the limits as discussed herein. Power extracted from theturbine, beyond that supplied by wind induced torque, will slow themachine down. The total energy extracted is the integral of this powerdifference. Also, the turbine has a lower limit on speed, below whichstall occurs. Thus, the total energy extracted must also be limited, sothat a minimum speed is maintained, with some margin. In one example, adynamic limit that is a function of the energy extracted may be used toaddress this aspect.

FIG. 3 is a graphical representation of an upper power limit in thecontrol technique of FIG. 2. The graph 100, shows X-axis as turbinepower (Pi), denoted generally by reference numeral 102, and Y-axis asupper limit ΔPmax₀, shown generally by reference numeral 104. A maximumupper limit, ΔP_(upper limit) is assigned in one exemplary embodiment,as shown by reference numeral 106. Thus, at any instant of time, theupper limit, ΔPmax₀ may be calculated as follows:ΔPmax₀ =ΔP _(upper limit){(Pi−P _(min))/(P _(rated) −P _(min))}  (1),wherein ΔP_(upper limit) is a hard upper limit, such as 10% of the ratedpower for example; P_(rated) is the rated power of the wind turbinegenerator, such as 1500 KW (Kilo Watts) for example, shown by referencenumeral 108; Pi is the turbine power reading at the time of transientcondition, shown generally by reference numeral 110; and P_(min) 112 isthe lowest turbine power below which the controller function is disabledfor any upward regulation.

A higher order limitation (i.e. not linear) is expected to optimizeperformance. In this example, the limit is further constrained as afunction of energy, as shown below in equation 2:ΔPmax(t)=ΔPmax₀ −k∫ΔPdt.  (2)Equation 2 illustrates a time dependent limitation of an upper powerlimit ΔPmax(t) based on power history (k∫ΔP dt) with k being a constantand t being time.

In another embodiment, the limit is a function of rotor speed. Thefollowing equation uses a linear function for purposes of example; morecomplex functions of turbine speed may be applied if desired.ΔPmax(t)=ΔPmax₀{(υ(t)−υ_(min))/(υ_(rated)−υ_(min))}  (3),wherein υ_(min) is greater than turbine minimum speed to provide margin,υ_(rated) is turbine speed at rated power and υ(t) is the instantaneousspeed of the wind turbine generator. It would be well appreciated bythose skilled in the art, that similar equations would apply to minimumΔP limits, but with appropriately adjusted signs and limits.

The system dynamics around an equilibrium point for a collection ofgenerators connected to a utility network are summarized below inequation (4), where ω and δ are vectors of electrical frequency andangle of the generators, relative to a common reference framerespectively; and M, D, K and C are the equivalent matrices of systeminertia, damping coefficients, torque constants and couplingcoefficients, respectively. The term ΔP(t, {circumflex over (δ)},{circumflex over (ω)}) is the supplemental power contribution from thewind turbine generator relative to the internal variables {circumflexover (ω)} and {circumflex over (δ)}.

$\begin{matrix}{\begin{bmatrix}\delta \\\omega\end{bmatrix} = {{\begin{bmatrix}0 & C \\{M^{- 1}K} & {M^{- 1}D}\end{bmatrix}\begin{bmatrix}\delta \\\omega\end{bmatrix}} + \begin{bmatrix}0 \\{\Delta\;{P\left( {t,\hat{\delta},\hat{\omega}} \right)}}\end{bmatrix}}} & (4)\end{matrix}$The response of a simple lumped inertia utility system 40 subject to adisturbance ΔP_(d) is given below in equation (5):M{dot over (ω)}+Dω+Kδ=ΔP(t,{circumflex over (δ)},{circumflex over(ω)})+ΔP _(d){dot over (δ)}=ω·ω_(b)  (5)where ω_(b) is the base frequency of the system.

One control strategy for generating the supplemental power input signalΔP is to add synchronizing and damping terms in phase with elements of{circumflex over (δ)} and {circumflex over (ω)}, respectively. Theseterms are used to help shape the system disturbance response to reducethe slope and magnitude of angular swings, and improve damping as willbe described in reference with FIG. 4-7. The control loop as shown inFIG. 2 takes advantage of the inherent overload capability of the windturbine electrical and mechanical system subject to limits on maximumtorque, power, ramp rates, and generator speed. Using dissipativeresistors and supplemental energy storage with closed-loop dampingcontrols minimizes interaction with turbine mechanical resonances.

FIGS. 4-7 illustrate the simulated disturbance responses for a windturbine system with and without the control loop as shown in FIG. 2.FIG. 4 and FIG. 5 show the frequency and power responses respectivelyfor an oscillatory system disturbance. In FIG. 4, the X-axis of thesimulated graph 114 is denoted by the reference numeral 116 andgenerally depicts time following the frequency deviation or disturbance(transient time) and the Y-axis, denoted by reference numeral 118,depicts the frequency of response. As illustrated, curve 120 is theresponse to the utility system disturbance without employing the controlloop of FIG. 2. The curve 122 on the other hand shows the frequencyresponse to the utility system disturbance when the control loop of FIG.2 is employed in the wind turbine system. The limits used were +/−3%limit on the supplemental power signal.

In the simulated graph 124 as shown in FIG. 5, the X-axis is denoted bythe reference numeral 126 and generally depicts the transient time andthe Y-axis, denoted by reference numeral 128, depicts the supplementalpower signal. As illustrated, line 130 is the zero supplemental powerinput when not employing the control loop of FIG. 2. The curve 132 onthe other hand shows the supplemental power input when the control loopof FIG. 2 is employed in the wind turbine system. The resulting systemfrequency behavior as shown by curve 122 in FIG. 4 and curve 132 in FIG.5 shows improvement in both power swing magnitude and damping.

Similarly FIG. 6 and FIG. 7 show disturbance responses for a monotonicsystem disturbance. In the simulated graph 134 of FIG. 6, the X-axis isdenoted by the reference numeral 136 and generally depicts the transienttime and the Y-axis, denoted by reference numeral 138, depicts thefrequency of response. As illustrated, curve 140 is the response to theutility system disturbance without employing the control loop of FIG. 2.The curve 142 on the other hand shows the response to the utility systemdisturbance when the control loop of FIG. 2 is employed in the windturbine system.

FIG. 7 similarly shows a simulated graph 144 illustrating another pairof responses, the X-axis is denoted by the reference numeral 146 andgenerally depicts the transient time and the Y-axis, denoted byreference numeral 148, depicts the supplemental power of the windturbine generator. As illustrated, line 150 is the zero supplementalpower input when not employing the control loop of FIG. 2. The response152 on the other hand shows the supplemental power input when thecontrol loop of FIG. 2 is employed in the wind turbine system.

Thus as described above, the transient power output or the supplementalinput signal may be implemented as a linear or non-linear relationshipof relative angle, relative frequency, and/or time, subject to multiplelimits. Limitations on the power amplitude and energy may also be used.Specifically, the amplitudes of the power limits, shown for example at+/−0.03 pu (per unit) in the FIG. 5 and FIG. 7 graphs are functions ofboth physical limitations of the electrical equipment, especially theconverter, and the physical limitations of mechanical equipment,especially the drive-train torque and speed.

It will be well appreciated by those skilled in the art that the controltechnique described herein may be utilized in a system for wind farmmanagement as well. Such a wind farm management system 200 is shown asan exemplary embodiment in FIG. 8. The wind farm management system 200includes a wind farm 210 having wind turbines 212, 214, and 216 operableto supply electrical power to a utility system 218. It will beappreciated by those skilled in the art that three wind turbines areshown for the purpose of illustration only, and the number may begreater based on the geographical nature and power requirements of anyparticular region.

Wind turbines 212, 214, 216 include turbine rotors 220, 222, 224, eachrotor having multiple blades, which drive rotors 220, 222, 224respectively to produce mechanical power, which is converted, toelectrical power by the generators 226, 228, and 230 respectively.Converters 232, 234, 236 are used to convert the variable frequencyoutput from the generators 226, 228 and 230 respectively into a fixedfrequency output. Power produced by generators 226, 228 and 230 may becoupled to a voltage distribution network or a collector system 238,which is coupled to the utility system 218. In the illustratedembodiment, a feeder 240 is used to couple power outputs of wind turbinegenerators 226, 228 and 230 for supply to the voltage distributionnetwork 238. In a typical application, the voltage distribution network238 couples power from multiple feeders (not shown), each feedercoupling power outputs of multiple wind turbine generators.

In one exemplary embodiment, the wind farm 210 includes a wind farmsupervisory controller 242. The supervisory controller 242 is configuredto communicate with individual wind turbine converters via communicationlinks 244, which may be implemented in hardware, software, or both. Incertain embodiments, the communication links 244 may be configured toremotely communicate data signals to and from the supervisory controllerin accordance with any wired or wireless communication protocol known toone skilled in the art. The supervisory controller 242 includes aninternal reference frame, and is coupled to the converters 232, 234,236, and is configured for modulating flow of power through theconverters 232, 234, 236 in response to utility system frequencydisturbances or power swings relative to the internal reference frame.The functionality of the supervisory controller 242 will be similar tothat of controller 32 described in reference to FIG. 2. In anotherembodiment, a plurality of controllers of the type shown in FIG. 1 areprovided to modulate the flow of power through each respectiveconverter.

It will be appreciated by those skilled in the art, that the windturbine system has been referred in the above embodiments as anexemplary power generation and power management system coupled to theutility system. Aspects of present technique are equally applicable toother distributed generation sources operable to supply power to theutility system. Examples of such sources include fuel cells,microturbines and photovoltaic systems. Such power managements systemswill similarly include converters, each converter coupled to arespective generation source and the utility system, and an individualor supervisory controller coupled to the converters. As explained hereinabove, the controller includes an internal reference frame configuredfor modulating flow of power through the converters in response tofrequency disturbances or power swings of the utility system relative tothe internal reference frame.

The controller, as described in the exemplary embodiments, provides adynamic control structure to modulate the torque or power component ofwind turbine generator output current as a function of the electricalangle (or relative frequency or time) between the utility system and theinternal wind turbine generator virtual-reference frame (“internalreference frame). The implementations of the above embodiments will alsoadvantageously facilitate the utility system independent operation ofthe wind turbine system, if desired, assuming high wind conditions andslow load dynamics.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A system for wind farm management, the system comprising: a pluralityof wind turbine generators operable to supply wind turbine power to autility system; a plurality of converters, each converter coupled to arespective wind turbine generator and the utility system; and acontroller comprising an internal reference frame of the wind turbinegenerators and configured for modulating flow of power through theplurality of converters in response to frequency disturbances or powerswings of the utility system relative to the internal reference frame,wherein the internal reference frame comprises a variable frequency thatis equal to a frequency of the utility system during steady stateconditions and that differs from the utility system frequency duringfrequency disturbances wherein modulating the flow of power comprisesproviding a supplemental input signal (ΔP) configured to increase ordecrease power output of the wind farm.
 2. The system of claim 1 whereinthe controller comprises a supervisory controller.
 3. The system ofclaim 1 wherein the controller comprises a plurality of individualcontrollers each coupled to a respective one of the plurality ofconverters.
 4. The system of claim 1 further comprising a differenceelement configured to obtain a relative frequency comprising adifference between the utility system frequency and the variablefrequency.
 5. The system of claim 1 wherein the supplemental inputsignal comprises a torque or power signal and is a function of at leastone of a relative angle, a relative frequency, or time.
 6. The system ofclaim 1 further comprising a limit function configured for limiting arelative angle, a relative frequency, a supplemental power or torquesignal, or combinations thereof.
 7. The system of claim 6 wherein thelimit function comprises limits that are operable as a function of atleast one of a physical limitation on the wind turbine system, a powerlimit, a torque limit, a current limit, an energy limit, or a windturbine generator rotor speed limit.
 8. The system of claim 1 furthercomprising an energy storage element, an energy consumer element orcombinations thereof, wherein the energy storage element, the energyconsumer element or the combinations thereof are coupled to theconverter.
 9. A system for power management, the system comprising: aplurality of distributed generation sources operable to supply power toa utility system; a plurality of converters, each converter coupled to arespective generation source and the utility system; and a controllercoupled to the plurality of converters, the controller comprising aninternal reference frame of the distributed generation sources, coupledto the plurality of converters, and configured for modulating flow ofpower through the plurality of converters in response to frequencydisturbances or power swings of the utility system relative to theinternal reference frame, wherein the internal reference frame comprisesa variable frequency that is equal to a frequency of the utility systemduring steady state conditions and that differs from the utility systemfrequency during frequency disturbances wherein modulating the flow ofpower comprises providing a supplemental input signal (ΔP) configured toincrease or decrease power output of the system.
 10. The system of claim9, wherein the internal reference frame is configured to output afrequency that may differ from the utility system during transientconditions.
 11. The system of claim 9 wherein the controller comprises asupervisory controller.
 12. The system of claim 9 wherein the controllercomprises a plurality of individual controllers each coupled to arespective one of the plurality of converters.